1. Field of the Invention
The present invention relates to a step-down switching DC-DC converter. More particularly, the present invention relates to a step-down switching DC-DC converter that is suitably applicable to power systems and devices using a solar cell or a battery as a power source, to systems and devices for charging and discharging a battery, to power systems and devices requiring low noise and the like.
2. Description of the Related Art
In an interplanetary probe using a solar cell as a power source, the supply voltage of the solar cell greatly varies when there is a great variation in the distance between the sun and the interplanetary probe. A power system of the interplanetary probe is required to step down the varying voltage of the solar cell to a desired voltage level before supplying the voltage to a load side. For this purpose, such a power system generally uses a switching DC-DC converter (or a switching regulator), which has low power loss and which generates less heat, as disclosed in Japanese Laid-Open Patent Application 1992-58757.
In addition, when the interplanetary probe is designed to measure electromagnetic or magnetic fields of a planet, a switching DC-DC converter is required to generate less switching noise.
However, the difficulty in using this kind of conventional, step-down switching DC-DC converter is that it will not feature low noise characteristics, as will be described below.
FIG. 1 is a circuit diagram illustrating the circuit structure of a typical buck converter in a typical step-down switching DC-DC converter. Referring to FIG. 1, as indicated in a block of ripple current ΔI, input current flowing through switch S has a pulse wave, and output current flowing along coil L has a triangle wave.
FIG. 2 is a circuit diagram illustrating the circuit structure of a typical low noise buck converter in a typical step-down switching DC-DC converter. Referring to FIG. 2, as indicated in the block of ripple currents ΔIL1 and ΔIC, both input current flowing along a coil L1 and output current flowing through a capacitor C can have a triangle wave.
However, the conventional step-down switching DC-DC converters shown in FIGS. 1 and 2 still have a problem in that they generate severe noise.
Specifically, according to the step-down switching DC-DC converter shown in FIG. 1, since input current flowing through switch S is a pulse wave, the switching frequency causes severe noise and thus a harmonic wave of the switching frequency also causes severe noise.
In the case of the step-down switching DC-DC converter shown in FIG. 2, both the input current flowing through the coil L1 and the output current flowing through the capacitor C are a triangle wave. Thus, the step-down switching DC-DC converter shown in FIG. 2 can cause less noise than the construction shown in FIG. 1 using the pulse wave. However, as indicated with the ripple currents ΔIL1 and ΔIC, currents flowing through input coil L1 and capacitor C also include the noise components of a switching frequency and its harmonic wave irrespective of whether not input coil L1 and intermediate coil L2 are electromagnetically coupled. Accordingly, it is impossible to eliminate ripple currents ΔIL1 and ΔIC (or achieve zero ripple currents).